Persistent diarrhea is a major world health problem. Unlike acute diarrhea, persistent diarrhea cannot be easily managed with oral rehydration therapy. Persistent diarrhea not only accounts of r a high proportion of diarrhea mortality in its own right, but it also contributes to malnutrition and compromised immune status which predispose to other potentially fatal diseases. For these elucidating the mechanisms, risk factors and optimal treatment of persistent diarrhea is a high research priority. One of the great controversies in he study of persistent diarrhea is whether or not enteroaggregative E. coli (EAggEC) specifically cause persistent diarrheal syndromes. These organisms have been implicated as agents of persistent diarrhea in several studies, yet in other studies they have not been found. One hypothesis to explain this observation is that the current definition of EAggEC my include non-pathogenic E. coli strains, which confound the analysis of diarrheal causality. Indeed, a volunteer study in healthy adults performed by the PI has demonstrated that fully virulent strains can be shown to have genotypic and phenotypic differences from non-pathogenic strains; such potential differences can be detected by DNA probes. The co-PI in this study, Dr. Alejandro Cravioto has conducted a landmark investigation, demonstrating a very high rate of EAggEC persistent diarrhea in a rural Mexican village, with a high association of EAggEC with persistent diarrhea. Under this proposal, the strains isolated during that study will be analyzed using new probes developed by the American PI in order to test hypotheses derived from lab and volunteer studies. In so doing, this collaboration will shed light on the fundamental pathogenetic, epidemiologic and clinical questions surrounding EAggEC and persistent diarrhea in general.